138 TEIJI HOSHINO 



8. Brain stem 



The medulla oblongata of each of the pigeons, both affected 

 and normal, with the attached half of the cerebellum was studied 

 in the successive serial sections from the distal end up to the 

 height of the oculomotor nucleus in the midbrain. Since the 

 normal structural relations of nuclei and tracts are not very 

 well known, I have described, of necessity, the normal structure 

 first before attempting to point out any alteration or difference 

 in the affected birds. 



The distal portion of the medulla oblongata. At this level of 

 the medulla oblongata, the gray substance of the posterior horn 

 increases in its horizontal width, the axis of the horn is directed 

 laterally, and it contains many myelinated fibers. The myelin- 

 ated fibers in the posterior horn converge ventromedially at 

 the neck of the horn, whence they run either into the funic- 

 ulus lateralis, or the anterior horn, while more proximally they 

 also enter the anterior commissure. 



The anterior horn is narrower than in the cervical cord; its 

 axis nears the midline, and it contains an abundance of fibers 

 which go to form a part of the thick anterior commissure. The 

 anterior commissure also receives fibers from the lateral fu- 

 niculus and from the posterior horn. These fibers in the anterior 

 commissure disappear in the opposite medial edge of the fu- 

 niculus anterior. Going more proximally, the anterior horn 

 becomes smaller and there appears the nucleus hypoglossus, 

 while laterodorsally to the central canal the beginning of the 

 vagus nucleus is seen. 



The fibers in the anterior funiculus correspond to the 

 fasciculus longitudinalis mcdialis of mammals. This has been 

 established by the early formation of myelin sheaths and the 

 pathway taken by these fibers to the midbrain (Brandis, '93). 

 Many fibers in the funiculus anterior and lateralis, which cross 

 in the raphe and run laterally and dorsally, correspond to the 

 internal arcuate fibers of mammals. The thick fiber bundle 

 which runs along the ventral periphery of the medulla forms 

 the external arcuate fibers and makes one of the important con- 

 nections with the cerebellum. 



